1. Field of the Invention
The present invention relates to a thermal expansile sheet.
2. Description of Related Art
Conventionally, a technique regarding a thermal expansile sheet is disclosed, for example, in U.S. Pat. No. 4,268,615. According to this technique, a desired image is first formed on the surface of a thermal expansile sheet using a material that is more optically absorptive than the thermal expansile sheet. Then, the surface is exposed to light. As a result of this, an area where the image is formed is heated and raised due to a difference in absorption.
Moreover, Japanese Unexamined Patent Publication No. 61-72589 discloses a technique in which an image is formed on an expandable recording member, using a material that is highly optically absorptive, by means of a thermal transfer method, and light is irradiated to the expandable recording member. Thus, an uneven pattern corresponding to the image is formed on the expandable recording body utilizing the optical absorption characteristics of the formed image.
The use of the above mentioned techniques makes it possible to make a desired uneven pattern on a sheet by a simple processing. The application of these techniques enables letters in Braille to be formed on a thermal expansile sheet.
However, when only Braille letters are formed on the sheet, it becomes impossible for those who have no knowledge of Braille to distinguish the top and bottom, i.e., the orientation of the Braille letters. For examples, Braille letters shown in FIG. 10A denote "Urashima Taro." The pattern in Braille shown in FIG. 10B is the pattern shown in FIG. 10A after being inverted and rotated 180.degree.. However, those who have no knowledge of Braille would assume that the Braille letters shown in FIG. 10B are also meaningful.
In addition, even when the Braille letters are properly oriented at the time of the preparation, the correct orientation of the Braille letters will become uncertain if the Braille label is temporarily stored before being attached or if it is dropped. This may result in the Braille label being used upside down. In fact, upside-down Braille signs are often seen at railway stations or the like.
To avoid this, it becomes necessary for those who have knowledge of Braille to check the orientation every time a label in Braille is used, which results in considerably laborious work.